"JC" virus has been isolated by Padget et al. (1971) from a human brain with progressive multifocal leukoencephalopathy (PML), a progressively fatal demyelinating disease often occurring as an opportunistic infection. In three instances, PML has been reported in patients after renal transplantation. The proposed studies are aimed chiefly at collecting data on the distribution, the light and electron microscopic features of the many diverse malignant tumors induced by this virus in the skin, general body organs and central nervous system of Syrian hamsters. There will be emphasis on the study of the experimental medulloblastomas, and their histogenesis, due to the importance of such tumors among childhood malignancies. Furthermore, there will be emphasis on the study of a pineocytoma, a tumor which also can occur in the human species and which has not been reported previously as having been induced in any other experimental set-up.